Note: Descriptions are shown in the official language in which they were submitted.
~ 5~ ~ 4 ~
This invention relates ~o the production of styrene,
and more particularly to treating an off-gas produced in a
process directed to the production of styrene from ethylben-
æene.
In the catalytic dehydrogenation of ethylbenzene to
produce styrene, the reactor effluent is normally cooled and
partially condensed to recover crude styrene product. The
off-gas from the cooling and partial condensation, which con-
tains primarily hydrogen as the non-condensable, has a high
content of aromatic hydrocarbons. In order to reduce such
aromatic hydrocarbon losses, such off-gas is generally com-
pressed, partially condensed and scrubbed with ethylbenzene,
followed by cooling and chilling to thereby reduce the aro-
matic content. In such processes, however, the vapors, which
contain as major components hydrogen, methane and carbon diox-
ide, are saturated with water and aromatic hydrocarbons;
namely, ethylbenzene. Such aromatics represent an important
loss of feedstock.
The present invention is directed to treating the
off-gas from a process directed to producing styrene from
ethylbenzene in order to provide an off-gas essentially free
of aromatic hydrocarbons.
In particular, the present invention provides a pro-
cess for treating an off-gas generated in a process for pro-
ducing styrene from ethylbenzene, said off-gas containing
hydrogen and aromatic hydrocarbons, which comprises finally
treating the off-gas which contains from 0.5% to 10% by volume
of aromatic hydrocarbons by contacting the off~gas at a tem-
pPrature of from 35F. to 125F. and a pressure of from 15
to 100 psia with an absorption oil for aromatics having a
5-volume percent distillation temperature of at lPast 400F.
t~
to absorb aromatic hydrocarbons present in the of-gas,
an~ providing a remaining off-gas essentially free of
aromatic hydrocarbons, which contains less than 0.2%, by
volume, of aromatic hydrocarbons and recovering aromatic
hydrocarbons absorbed by the absorption oil.
In accordance with the present invention, the off-
gas from the process for producing styrene from ethylben-
zene is finally treated with a heavier oil which absorbs
aromatics to thereby provide a remaining off-gas which
is essentially free of aromatic hydrocarbons.
The oil for absorbing aromatic hydrocarbons
has a 5-volume percent distillation temperature of at
least 400F, with the 5-volume percent distillation tem-
perature generally being at least 600F.
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~5~ 7
. .
In most cases, the 95-volume percent distillation temperature does not
exceed about 1000F. The abss~rption oil may be any one o~ a wi~lc variety
of absorption oils which are kno~rn 'o be effective for absorbing aromatics
such as ethylbenzene, with such absorption oil being characterizecl by thc
5 5-volume percent boiling characteristics hereinabove noted. In most
cases, the absorption oils which are effective for absorbing aromatics
are of an aromatic nature, although such absorption oiLs may contain
some aliphatic components. As representative e~amples of suitable
absorption oils, there may be mentioned gas oils, fluid catalytic
10 cracking cycle oils, lube oil extraction raffinates and the like. A pre-
ferred absorption oil is the heavy byproduct from the process for pro-
ducing ethylbenzene by alkylation of benzene. Such heavy byproduct
generally contains as principal components polyethylbenzenes and
diphenylethane.
The off-gas which is subjected to scrubbing with the heavier
absorption oil has an aromatics content of from 0. 5 to about
10-volume percent, with the other principal components being hydrogen,
methane, water vapor and carbon dioxide. The aromatics present in
the off-gasareprincipally comprised of ethylbenzene and the off-gas may
2G further include one or more of benzene, toluene and styrene.
The scrubbing with the heavier absorption oil is employed as
a final treatment of the off-gas in order to essentially eliminate aromatics
therefrom. Consequently, prior to contact with the heavier absorption
oil, the off-gas has been tréated to reduce the aromatic content thereof
25 by one or more of chilling and¦or ethylben~ene scrubbing.
1847
The invention will be further described with respect to preferred
-- embodiments thereof illustrated in the accompanying drawings, whcrein:
Figure 1 is a simplified schematic fLow diagram of a prefcrred
embodiment of the present invention;
Figure 2 is a simplified schematic flow diagram of a modifi-
cation of the embodiment of Figure 1; and
Figure 3 is a simplified schematic flow diagram of a urther
modification of the embodiment of Figure 1.
Referring now to IFigure 1 of the drawings~ ethylbenzene fresh
feed, in line 10, and a further portion OI ethylbenzene fresh feed, including
absorbed aromatics, in line 11, obtained as hereinafter described,are
introduced into a styrene production zone, generally designated as 12
wherein the ethylbenzene is catalytically dehydrogenated to produce
styrene, by a procedure known in the art. As known in the art, the
feed to zone 12 generally also includes steam. In general, the styrene
production is operated at a temperature in the order of from 500F to
1500FJ and at a pressu--e in the order of from 2 to 25 psia.
~ styrene production effluent is withdrawn from the styrene
production zone 12 through line 14 and introduced into a suitable cooling
zorle, schematically generally indicated as 15 (which may contain onc or
more cooling stages) in orcler to cool the effluent gas and conclense
crude styrene product, which contains in add ition to styrene, ethy l-
.
benzene~ and one or more of toluene and benzene. Crude styrene
product is withdrawn from cooling zone 15, through line 16 for intro-
duction into a separation and recc)very zone (not shown).
Uncondensed gas, which is at a temperature in the order of
from 60F to 150F, and at a pressure of from 2 to 25 psia, is with-
drawn from cooling zone 15 through line 20, and such gas contains
hydrogen, methàne, carbon dioxide, as welL as water vapor ancl aro-
matics, (ethylbenzene and some styrene and/or toluene and/or benzene).
The gas in line 16 is compressed by the use of a compressor, sche-
matically generally indicated as 17, and the compressed gas in line 18
is introduced into a cooling zone, schematically generally indicated as
19, which may contain one or more cooling stages in order to reduce the
temperature of the gas and condense aromatics therefrom. Condensed
aromatics are recovered frorn cooling zone 19 through line 21, and are
combined with the crude product in line 16 for introduction into the
styrene separation and recovery zone (not shown).
The remaining gas withdrawn from the cooling zone 19 through
line 22, which is generally at a temperature in the order of from 75F
to 125~F, and at a pressure in the order of from 15 ~o 100 psia and which
still contains aromatics, is introduced into a scrubbing zone, generally
indicated as 23, wherein the gas is countercurrently contacted with a
portion of the ethylbenzene freshfeed to the styrene production, intro-
duced through line 24. The scrubber 23 contains suitable meansJ in
order to increase gas-liquid contact~ e. g. packing 25, and as a result
of such contact, further aromatics are scrubbed from the gas.
~51B47
Ethylbenzene, containing absorbed aromatics, is withdrawn
from scrubber 25 through line 11 for introduction into the styrenc
production zone 12.
The gas ~vithdrawn from scrubber 23 through Line 27 is
5 at a temperature in the order of from 35F to 150F, and a pressure
in the order of from 15 to 100 psia. Such gas still contains some
aromatics, and in general, as hereinabove described/ the aromatic
content of the gas is in the order of from 0. 5% to 10%, by volume.
The gas in line 27 is introduced into a further scrubber 28,
which includes suitable gas-liquid contacting means, shown as bed 29
wherein the gas is contacted wLth an absorption oil int:roduced through
line 31. The absorption oil introduced through line 31 is a heavy oil
of the type hereinabove described, which is effective for absorbing
aromatic hydrocarbons from the gas introduced through Line 27. As
hereinabove described, the absorption oil is preferahly the heav~T by-
product from a process for producing ethylbenzene, ~nd is characterized
by a 5-volume percent distillation temperature of at least 400F,
preferably at least 6û0:F, and a 95~volume percent distillation tem-
perature of no greater than 1000F.
The scrubber 28 is operated at conditions to remove essentially
alL of the aromatics present in the gas introduced through line 27; i.e.
the aromatic content of the scrubbed gas is less than 0. 5%,most
generally less than 0 . 2~o and preferably less than 0. 05~, all by volume.
In general, scrubber 28 is operated at a temperature in the order of
from 35F to 125F, and a pressure in the order of from 15 to 100 psia.
347
. . .
~n off-gas, essentially free of aromatic hydrocarbons, which
generally contains 90-volume percent or more of hydrogen, is re-
covered from scrubber 28 through linc 32. Such oîf-gas may be
burned, or in some cases, may be further treated to recover hydrogcn
therefrom.
Rich absorption oil withdrawn from scrubber 28 through line
33 is heated in exchangers 34 and 35 prior to being introducetl through
line 36 into an absorption oil stripper, schematically generally Incli-
cated as 37, with the stripper being provkled with suitable gas-Liquid
contacting means, such as a bed 38.
Stripper 37 is operated at temperatures and pressures to strip
absorbed aromatics from the absorption oil, with the stripper 37 pre-
ferably beiIlg operated by the use of a suitable stripping gas., such as
steam, provided through line 39. Although steam is preferred" it is
to be understood that stripping gases other than steam may also be
employed. Thus, for example, Instead vf steam, there may be used:
nitrogen, methane, carbon dioxide or the like. The stripping gas can
be a combination of the above. It is also to be understood that the
stripper may be operated without a stripping gas, although this is less
preferred. ALternatively, the stripper may be replaced with a flash
drum when higher boiling absorption oils are used.
In general, stripper 37 is operated at a temperature in the
order of from 125~F to 300~F, and at a pressure in the order of from
1 to 25 psia.
Stripped arornatic components are withdrawn from stripper 37
-- 6 --
through line 41, and such stripped components are introduced into
the cooling zone 15 to condense such aromatics for recovery with
the crude product in line 1fi. It is to be understood that the stripped
aromatics can be condensed in a separate zone, although condensation
5 in zone 15 is preferred.
Lean absorption oil is withdrawn from stripper 3rt through
line 42, and a portion thereof is purged through line 43, with the re-
maining portions being cooled in exchanger 3D~, by indirect heat transfer
with rich absorption oil, and further cooled in exchanger 40, prior to
10 introduction into scrubber 28 through line 31. Suitable make-up
absorption oil is provided through line 44.
Thus, in accordance with the embodiment, the off-gas from the
styrene production, which still contains some aro~ tic hydrocarbons,
is finally treated with a heavier absorption oil to recover such aromatic
15 hydrocarbons for ultimate reuse in the process.
Referring to Figure 2 of the drawings, there is shown a modi-
fication of the embodiment described with respect to Figure 1. Thus~
in accordance with the embodiment of ~igure 2, off-gas from cooling
stage 19, which is in line 229 is cooled in a chiller, schematically
20 generally indicated as 101, to condense a portion of the aromatics
contained therein, wi~h the condensed portion being recovered in line
102 for introduction into the crude styrene recovery system ~not shown),
or in the alternative to the styrene production reactor.
The uncondensed portion of the gas9 in line 103, which is
generally at a temperature in the order of from 32F to 100DF" and
L7
at a pressure in the order of from about 15 to 100 psia is introduced
-- into scrubber 104, including suitable gas-liquid contact means9 in
the form of a bed 105. In scrubber 104, the gas is contacted with a
heavy absorption oil, of the type hereinabove described introducecl
5 through line 130 for absorbing aromatics which remaLn in the gas
introduced through line 103. The scrubber 104 is operated at con-
ditions to produce a gas which is esserltially free of aromatics (as
hereinabove described), with the essentially aromatics free gas, ~vhich
primarily contains hydrogen and methane, being recovered through
10 line 105 for further processing by burning or hydrogen recovery, as
hereinabove described.
The scrubber 104 is generally operated at a temperature in
the order of from 35F to 125F, and at a pressure in the order of from
15 to 100 psia.
Rich absorption oil is withdrawn from scrubber 104 through
line 106 and heated in exchangers 107 and 108 prior to introduction
thereof into a str ipp ing column 1 0 9 through line 110, The s tr ipp ing
column 109 is provided with suitable gas liquid corltact means, for
example, a packed bed 112. The stripper 109 is provided with a
20 suitable stripping gas, such as steam~ through line 114; however, as
hereinabove described with respect to the embodiment of Figure 1,
other stripping gases could be used, or in the alternative, the stripper
could be operated without the use of a stripping gas or replaced with a
flash drum. The stripper 109 is operated to strip absorbed aromatics
25 from the absorption oil, with the stripped aromatics being witlldrawn
.
from stripper 109 through line 115 for introduction into the cooling
zone 15, as hereinabove described with respect to the embodimcnt
of Figure 1, or in the alternative condensed separately. In gencral,
the stripper is operated at a temperature in the order of from 125F to
300F, and at a pressure in the order of 1 to 25 psia.
Lean absorption o il is withdrawn from str ipper 1 0 9 through
line 116, with a portion thereof being purged through line 117. Lean
absorption oil is then cooled in exchanger 107 by indirect heat transfer
with rich absorption oil, and further cooled in exchanger 118 prior to
introduction into the scrubber 104. Make-up absorption oil is provided
through line 121.
Thus, in accordance with the embodiment of Figure 2, the ethyl-
benzene scrubber has been eliminated, and replaced by a chillerJ with
the off-gas being finally treated, as hereinabove described with respect
to the embodiment of Figure 1 by use of a heavier absorption oil in
order to provide an off-gas which is essentially free of aromatics.
A further modification of the embodiment of Figure 1 is shown
in Figure 3 of the drawings. Referring to Figure 3, off-gas in line 22
is introduced into the ethylbenzene scrubber 23?, which includes suitable
gas-liquid contact bed 251J wherein the gas is countercurrently contacted
with ethylbenzene, as an absorption liquid, introduced through line 24'.
The scrubber 23t is operated as hereinabove described with respect to
the embodiment of Figure 1, with the ethylbenzene, containing absorbed
aromatics being recovered therefrom through line 11 ' for recycle to the
styrene production.
~53 ~39L7
. . .
The gas withdrawnfrom scrubber 23' through line 201,
which is generally at a temperature in the order of from 35 to 150F
and a pressure in the order of from about 15 to 100 psia, is coole~l in
exchanger 202, and further chilled in chiller 203 in order to condense
5 further aromatics from the gas, with the condensed aromatics bcing
withdrawn from exchanger 202 through line 204 and from chiller 203
through line 205 The condensed aromatics recovered through lines
204 and 205 are recycled to the styrene production reactor or in the
alternative to the recovery system. The gas withdrawn from chiller
203 through line 206, which is at a temperature in the order of from
32F to 100F, and at a pressure in the order of from about 15 to
100 psia is heated in exchanger 202 by indirect heat transfer with the
gas in line 201, and the heated gas in line 207 is introduced into an
additional scrubber 208, which includes suitable gas-liquid contact
1 5 means in the form of a bed 2 09 wherein the gas is countercurrently
contacted with absorption oil introduced through line 211 for absorbing
aromatics. The absorption oil employed in line 211 is a heavier ab-
sorption oil of the type hereinabove described. The scrubber 208 is
operated at conditions to absorb essentially all of the aromatics present
in the gas introduced through line 207, with a gas, essentially free of
aromatics, as hereinabove described, being recovered -frorn scrubber
208 through line 209 for burning and/or hydrogen recovery as herein-
above described. In general, the scrubber 208 is operated at tempe-
rature in the order of from 35F to 125F, and at a pressure Ln the
order of from 15 to 100 psia.
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4'7
Rich absorption oil withdrawn from scrubber 208 throu~h
line 212 is heated in exchangers 214 and 215, with the heated rich
absorption oil in line 216 being introduced into a stripper 217, whicll
includes suitable gas-liquid contacting means such as a bed 218. Thc
stripper 218 is preferably providecl with a stripping gasJ such as
steam through lLne 219, although, as hereinabove described, other
stripping gases may be employed or the stripper 217 may be operated
without a stripping gas or replaced with a flash drum. The strippcr
217 is operated at conditions to strip absorbed aromatics from the
absorption oil, with the stripped aromatics being withdrawn from the
stripper Z17 through line 221 for introduction into the cooling section
15 as hereinabove described with respect to Figure 1, or in the alter-
native condensed separately. In general, stripper 217 is operated at
temperatures in the order of from 125~F to 300F, and at a pressure in
the order of from 1 to 25 psia.
Lean absorption oil is withdrawn from stripper 217 through
line 222 and a portion thereof is purged through line 22S. The lean
absorption oil is cooled in exchanger 214 by indirect heat transfer with
the rich absorption oil in line 212, and is further cooled in exchanger 230
prior to introduction into the absorption column 208 through line 211.
Make-up absorption oil is provided ~hrough line 216.
Thus, in accordance with the embodiment of Figure 3, prior
to the final treatment with the heavier absorption oil, and subsequent
to ethylben~ene scrubbing, the gas is cooled to condense additional
aromatics therefrom.
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1l~5~B47
Although the invention has been particularly described witl
respect to embodiments thereof as illustrated in the accompanying
drawings, it is to be understood that the invention may be practised
other than as particularly described with respect to the embodimcnts.
5 Thus, for example, the various heat transfer stages may be effecte(l other
than as particularly described.
The present invention is particularly advantageou;, in that it
is possible to recover essentially all of the aromatics which are present
in l;he off-gas generated in a styrene production process. The increased
10 recovery of aromatics improves the economics of the process. In
addition, if hydrogen is to be recovered from the off~gas, by increasing
the hydrogen concentration, such hydrogen recovery is enhanced.
_ 12 ~
